High performance electrochemical storage devices can be divided into two main categories: batteries and supercapacitors. Batteries traditionally deliver high energy, but suffer when it comes to power delivery. On the other hand, supercapacitors are noted for having large power densities at the sacrifice of delivering sub-par energy. In order to meet future energy demands, the gap between batteries and supercapacitors must be bridged by creating devices that concurrently deliver both high energy and power.
FIG. 1 depicts a conventional pore structure. Three-dimensional porous composite electrodes are currently used in commercial applications, in particular for many battery chemistries including Nickel Metal Hydride (NiMH), Lithium Ion and Lead Acid. Most NiMH batteries utilize a nickel foam cathode to increase electrical conductivity. These nickel foams generally have 80-180 holes per inch, with an average hole diameter of ˜0.25 mm. A layer of electrolytically active materials is then attached to the nickel foam in a slurry addition, creating a 3D composite electrode. This method was first patented by General Electric Company in 1967 with a desirous pore structure that contains about 100 pores per inch or pore sizes of about 250 μm. The Mitsubishi Materials Corporation in 1996 improved upon the initial design through considering pores on the range of 60 to 700 μm, thus slightly reducing the length that ions and electrons need to flow. This foam type architecture can be utilized in a variety of materials including nickel, copper, carbon based materials, and a range of other electron conducting phases.
In order to meet growing energy demands, there have been many attempts to improve foam morphology, through creating materials that have a more symmetric pore architecture and through reducing overall pore dimensions, enabling low-resistance transport paths for ion and electrons throughout the structure. These attempts all have drawbacks. FIG. 2 displays various different conventional architectures. Section 200 depicts a lithography defined microstructure, section 205 depicts a nanofoam architecture, section 210 depicts a nanowire based array, and section 215 depicts an inverse opal structure. Multi-beam laser lithography has been used to create nanostructures with a face centered cubic lattice and uniform pore distribution with pore sizes of about ˜800 nm. The nanofoam architecture is composed of interconnecting fibers ranging from 100-1000 nm in diameter and porosity of 99%. Nanowire based architectures consist of aligned nanowires with uniform diameters that can range from ˜1 nm to ˜500 nm, and lengths as long as 1 μm. The inverse opal structure consists of pores that range from ˜200 nm to ˜10 μm depending on the initial spheres used to create the template, and offers uniform pore distribution.
Prior art methods suffer from major disadvantages. Regarding foams, the electrolytically active phase is deposited via a slurry addition, which fills the pores and does not allow for a percolating pathway for the electrolyte. The processing of foams, in combination with the large pore sizes (˜0.25 μm) and pore distribution, creates long pathways for ions and electrons to transport, limiting the achievable power densities to that of traditional batteries.
Lithography defined structures have a uniform pore size distribution, but the pore size itself is not uniform. This limits the active material that can be deposited while also ensuring a percolating passage for each phase. Moreover, this method requires expansive techniques such as multi-beam laser interference lithography and argon plasma sputtering. The processing step also inhibits the thickness of the electrode to ˜4 μm, limiting the total energy that can be stored in the device.
Regarding nanofoams, the diameters of the fibers that comprise the structure range over an order of magnitude producing pores and a pore distribution that are not uniform. Further deposition of an active material could then cause a blockage, not allowing the electrolyte to freely flow within the structure.
Regarding nanowires, due to the small diameter of these nanowires, they have a tendency to colligate together, creating non-uniform pores and pore distribution. As with other conventional methods, this can increase the resistive paths for ions and electrons, which in turn will limit the power density. Furthermore, the distance between the wires is very small, limiting the amount of active material that can be deposited and in turn, the energy density.
Regarding inverse opal structures, similar to lithography defined structures, the pore structure is non-uniform, limiting the amount of active material that can be deposited while still keeping a continuous path for the electrolyte. Additionally, this processing technique can only produce electrodes with a thickness of up to ˜15 μm, limiting the total amount of stored energy. This method also requires extensive techniques such as evaporative deposition and electrodeposition.